Add a link to the 'Repository Storage Types' doc page

Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/administration/index.md

@@ -139,6 +139,7 @@ Learn how to install, configure, update, and maintain your GitLab instance.
 
 - [Repository checks](repository_checks.md): Periodic Git repository checks.
 - [Repository storage paths](repository_storage_paths.md): Manage the paths used to store repositories.
+- [Repository storage types](repository_storage_types.md): Information about the different repository storage types.
 - [Repository storage rake tasks](raketasks/storage.md): A collection of rake tasks to list and migrate existing projects and attachments associated with it from Legacy storage to Hashed storage.
 - [Limit repository size](../user/admin_area/settings/account_and_limit_settings.md): Set a hard limit for your repositories' size. **(STARTER ONLY)**